271 Chapter 11 Maximizing the Lifetime of an Always-On WSN Application
Santosh Kumar, Anish Arora, and Ten H. Lai
5. The average number of times a data packet is transmitted for reliable
delivery across a single hop is 1/0.7 = 1.43. This is because the per-hop
reliability of data packets was 0.7 in ExScal7. If we assume the probability
of losing a packet is independent and identically distributed, the expected
number of times a packet needs to be transmitted for successful delivery is
1/p (expectation of geometric distribution [22]), where p is the probability
of success in each transmission8.
6. Periodic control messages such as routing and time synchronization updates
are never retransmitted.
7. We assume a fixed packet length of 36 bytes, which is the maximum length
of a packet in B-MAC [20] (the MAC protocol used in ExScal), and that
it takes 20 ms to transmit a packet. In practice, event detection messages
are very short, and therefore the packet length will be smaller and so will
the transmission time.
The assumptions for any other always-on application will mostly be along the
lines of the above assumptions with some changes specific to that application.
Table 3. Number of sensors detecting an intruder type in the ExScal application.
Intruder # of PIRs # of Acoustic # of Magnetometers
SUV 30 30 5
Person 10 None None
4.2 Parameters for Lifetime Analysis
In this section, we define the parameters used in analyzing the lifetime of an
always-on WSN and derive their values in the ExScal application.
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